1. Field of the Invention
This invention relates to protective relaying systems for multiphase electric power distribution networks, and particularly to such systems in which protective relays at terminals at opposite ends of a protected line segment exchange instantaneous current phase information over a communications link for use in identifying faults in the protected segment and for generating trip signals for circuit breakers at each terminal which isolate the faulted line segment.
2. Background Information
Electric power transmission lines are protected against faults by protective relay systems which operate circuit breakers to isolate the faulted segment of the line. Such systems include a protective relay at each terminal of the protected line segment which analyze line currents and in some systems voltages to determine the location of a fault and to trip the circuit breakers at the terminals to isolate a fault determined to be between the terminals.
In one type of protective relaying known as phase comparison relaying, relays at the two ends of a protected line segment generate for each phase a square wave signal representative of an instantaneous comparison of the magnitude of the current with a keying level indicative of a fault condition. These square wave signals for each phase are transmitted over a communications link to the other terminal where they are compared to local square wave signals generated by instantaneous comparisons of the local phase currents with other threshold signals. Coincidence of the local and remotely generated square wave signals indicate an internal fault and generate trip signals which operate circuit breakers to isolate the protected line segment. Examples of such phase comparison relaying systems are disclosed in U.S. Pat. Nos. 3,986,079 and 4,053,940.
Typically, the remote square wave signals are transmitted between the terminals at opposite ends of the protected line segment over a two-way communications link by modulating a carrier signal. While other modulating techniques could be employed, U.S. Pat. No. 4,380,746 discloses a pulse period modulation technique which is used with the pulse period demodulation technique disclosed in U.S. Pat. No. 4,510,453 to provide a simple modulation-demodulation scheme which meets the real time requirements of phase comparison relaying.
Conventionally, three separate two-way channels, one for each phase, are used to exchange the square wave information between terminals. Typically, a telephone modem in which the modulated carrier is transmitted as tones is used in the communication link.
Phase comparison relaying schemes may also generate a direct transfer trip signal which is a direct command to trip the remote circuit breaker independent of the phase relation of the currents of the two terminals. These signals may be generated for instance for maintenance purposes, or to coordinate tripping of the circuit breaker with circuit breakers in other protected line segments of the distributing network.
U.S. Pat. No. 4,464,697 discloses a technique for transmitting this direct transfer trip signal over the available communications channels by disconnecting the square wave signals from the modulator and substituting a separate tone generator unique to the direct trip signal.
While the current systems provide satisfactory protective relaying, there is a need for a phase comparison relaying system which requires fewer communications channels.